It is known to provide multiple antenna wireless communication systems which exploit multiple spatial channels in the transmission medium between the transmitter and the receiver, to simultaneously transmit multiple different information streams, or to simultaneously transmit multiple copies of the same information redundantly. In the first case the capacity is increased, and in the second case the quality or robustness can be increased. Such multiple antenna wireless communication systems are known as MIMO (multiple input multiple output) systems, where there are multiple antennas at both ends. They are known as MISO (multiple input single output) where there is only a single antenna at the receiver. The multiple data streams can be referred to as MIMO channels or spatial channels, to distinguish from frequency or coding channels. Thus where different information is sent on different spatial channels, this is referred to as spatial multiplexing, and where the same information is sent, this is referred to as spatial diversity or transmit diversity.
The transmitted data streams may experience different channel conditions (e.g. different fading and multipath effects) and so have different signal to noise ratios (SNRs). Since the channel conditions typically vary with time, the data rate supported by each channel may vary with time. If the characteristics of each MIMO channel (e.g. SNRs for the data streams) are known at the transmitter, then the transmitter may be able to determine a particular data rate and coding and modulation scheme for each data stream adaptively with closed loop control to a given packet error rate. However, for some MIMO systems, this information is not available at the transmitter, so these are open loop systems.
Per-Antenna Rate Control (PARC) is a multiple path, multiple-antenna MIMO technique that has been proposed within the well known 3rd Generation Partnership Project (3GPP). Within 3GPP, PARC is applied to code division multiple access (CDMA) systems but the method is also applicable to systems without spreading or employing other transmission techniques, including orthogonal frequency division multiplexing (OFDM). FIGS. 1 and 2 show a transmitter and receiver respectively to show how the PARC scheme operates.
As shown in FIG. 1, the bit-stream to be transmitted is split by a demultiplexer 10 into a number of streams each carrying different information, each of which has (potentially different) modulation and coding applied as well as interleaving. Each stream has forward error correction 20, 21, 22, interleaving and coding 30, 31, 32, and modulation 40, 41, 42. The streams are then transmitted via separate antennas 50, 51, 52.
At the receiver (see FIG. 2) a number of antennas equal to or greater than the number of transmit-antennas is typically used. The signal at each receive-antenna 11, 12, 13 is a combination of the signals transmitted from each transmit-antenna 50, 51, 52. The receiver can apply an algorithm such as Minimum Mean Square Error (MMSE) estimation shown by item 15, or MMSE plus Successive Interference Cancellation (SIC) in order to estimate the symbols of each transmit stream. The receiver may also provide feedback to the transmitter of measurements of the channel quality, for example a signal to noise plus interference ratio (SNIR) measurement for each transmitted stream. The transmitter may use this information in making decisions to adapt the appropriate modulation and coding for each stream. Following the MMSE, the receiver processes the streams from each antenna to reverse the processing in the transmitter, so demodulation 25, 26, 27 is followed by deinterleaving 35, 36, 37, followed by a viterbi type decoding 45, 46, 47, and remultiplexing (merging) of the streams 55.
PARC can achieve spatial multiplexing gains, i.e. the simultaneous transmission of multiple data streams using the same time and frequency resources but different ‘spatial’ resources (i.e. multiple transmit antennas). These simultaneous transmissions might be destined for a single receiving unit, or to different receiving units. In the latter case this is referred to as Spatial Division Multiple Access (SDMA). PARC may be considered as a refinement of the original BLAST family of spatial multiplexing techniques that includes rate adaptation of the spatially multiplexed sub-streams.
Spatial multiplexing schemes aim to maximise the data throughput and achieve as near as possible the available capacity of the wireless channel.
An alternative strategy is spatial diversity, also called transmit diversity, as mentioned above. Although the gains from both spatial multiplexing and transmit diversity are maximal in a full rank channel, such diversity gains are less sensitive to the magnitude of the individual singular values than spatial multiplexing gains and are achievable in a wider set of practical channel scenarios. “Full rank” is defined as follows. For a single carrier, or a single subcarrier in an OFDM or other multi-carrier system, a flat-fading channel may be expressed as an MR×MT matrix, relating the signals at the MR receive antennas to the symbols from the MT transmit antennas. The non-zero singular values of the channel matrix give a measure of the number and quality of the spatial sub-channels that are available for spatial multiplexing. The maximum number of non-zero singular values is min(MR,MT) and in this case the channel is “full rank”.
Briefly, spatial diversity consists of transmitting the same data stream via multiple antennas and/or receiving this data stream via multiple receive-antennas. This provides the receiver with multiple copies of the wanted data stream, with each copy usually experiencing different channel conditions. The receiver can combine the different sub-streams in an optimal way in order to provide the best estimate of the transmitted data stream.
Typically, diversity schemes utilise space-time coding of the data stream in order to produce streams for each transmit antenna. The to some extent conflicting, aims of space-time coding are to achieve full diversity for improved communication performance, plus orthogonality for low complexity decoding, whilst retaining the information rate as high as possible.
Currently proposed MIMO systems typically provide either spatial multiplexing or spatial diversity, and are therefore only optimal in a certain subset of scenarios, i.e. in a certain subset of the full set of wireless channels that might be experienced. Current proposals to simultaneously achieve both gains suggest                space-time codes that offer a predetermined degree of both diversity and spatial multiplexing [see Texas Instruments, “Double-STTD scheme for HSDPA systems with four transmit antennas: Link level simulation results”, TSG-R WG1 document, TSGR1#20(01)0458, 21st-24th May, 2001, Busan, Korea], and also        a ‘switched’ scheme that effectively implements both diversity and spatial multiplexing schemes in both the transmitter and receiver and switch between these according to some criterion [see IST-2003-507581 WINNER, “Assessment of Advanced Beamforming and MIMO Technologies”, D2.7, February 2005].Another system known from US patent application 2003/0013468 shows using a scramble code to reduce the auto-correlation between time delayed versions of the same data stream. The scrambling will be the same for each of the streams transmitted from a particular transmitter.        